Memory cells, for example, in a Flash memory device, may be read by sampling the cells with a single initial set of decision thresholds for “hard” decoding. A hard decoder may define each bit of each memory cell to have an exact and definite value, for example, either zero or one. For example, if voltage is induced across a bit using the initial set of hard decision thresholds, the bit may be considered to be definitely a one (otherwise the bit may be considered to be definitely a zero).
In addition to the hard bit information defining the bit value (e.g., 0 or 1), “soft” decoders and “erase” decoders may provide additional “soft” bit information, for example, providing a measure of the certainty or reliability that the bit value is correct. To obtain the soft bit information, the memory cells may be sampled or read repeatedly using, not only the initial set of hard decoding threshold(s), but also multiple additional soft decoding thresholds surrounding each hard decoding threshold to determine how close the voltage is to the hard decoding threshold. Similarly, an erase decoder may define “erase bits,” which may indicate whether the hard bit information is reliable or not by sampling the cells in a voltage window using a pair of thresholds positioned at an upper and lower voltage limit for each hard decision threshold.
Repeatedly sampling the same set of cells at staggered thresholds may provide additional soft information about the reliability of the values read at those cells to reduce decoding errors and enhance read reliability. However, since each sampling operation uses a separate read cycle, repeatedly sampling the same set of cells in soft or erase decoding may use significantly more time and resources than the single sampling operation used in hard decoding.
There is a growing need in the art for systems and methods to reduce the duration of multiple sampling read operations for soft decoding and erase decoding.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.